The invention relates to a device for optical detection of the road and traffic from a road vehicle, by electronic processing and evaluation of the image signals from a video camera mounted in an elevated part of the vehicle (for example at the level of the top edge of the windshield of the vehicle) , with the slope angle E that the optical axis of the camera lens forms with the plane of the road being less than, but approximately equal to, half the total aperture angle (2.theta.) of the lens.
A known system of this kind, disclosed in European Patent Application 89 309 876.4 (publication number 0 361 914 A2), determines the path of the road edges by detecting sections of straight lines in the image generated by the video camera, which edges converge with increasing distance from the vehicle. Systems of this kind that permit rapid "real time" image processing are used for example as "doze alarms" that trigger a warning signal for the driver if the vehicle's orientation relative to the road deviates from the desired values. Theoretically, such a system also permits intervention in the power steering of the vehicle to maintain an orientation relative to the road and the vehicle's position on the road if the driver fails to react, and makes it possible to control automatic braking and stopping of the vehicle, as well.
The effectiveness with which these functions can be fulfilled improves as the distance from the vehicle at which the road is "seen" decreases, and as the distance at which the road can be detected with sufficient position resolution increases. These current requirements, to achieve the best possible compromise, can be met, but not satisfactorily, by using a camera with a lens having an average viewing angle that is slightly smaller than that of a normal lens. Better results, but by no means satisfactory, can be achieved at the price of a considerable increase in cost, by using two video cameras, one of which is associated with the near field in front of the vehicle and is fitted with a wide-angle lens, and the other of which is associated with the distant range and is equipped with a telephoto lens.
Hence, the goal of the present invention is to provide an improved system of the type described above which can achieve a position resolution for image evaluation in both the extreme near range of the vehicle and also in areas on the road that are relatively distant, using a single video camera.
This goal is achieved according to the invention by providing the video camera with a wide-angle lens whose aperture angle (2.theta.) is at least 6.degree., and by positioning the photosensitive sensor area of the video camera to receive that part of the image which corresponds to a distance range that is between a certain minimum distance from the camera and the farthest distant areas of the acquired area. In the system according to the invention, the imaging scale for areas far from the vehicle is larger by a factor M than it is for areas close to the vehicle. A value of M can be calculated by means of the following expression: ##EQU1## in which .epsilon. designates the slope angle of the optical axis of the camera lens relative to the road. (The value of M thus calculated is somewhat smaller than the value which is yielded by an alternative method described hereinafter.)
Thus, the camera lens exhibits the imaging properties of a telephoto lens for portions of the acquired area that are distant from the vehicle and exhibits the image-field widening properties of a wide-angle lens only for the area that is close to the vehicle. By allocating the photosensitive sensor area of the video camera to that part of the image that corresponds to a distance range from a preset minimum to the far distant portions of the acquired area, the (limited) image resolution of the photosensitive sensor area of the CCD camera can be used optimally for the surveillance of areas of interest.
If the aperture angle of the lens is at least 90.degree., the lens can be set with its optical axis inclined 45.degree. or more relative to the road. Thus, the imaging scale M for the distant areas of the road is larger by a factor of 2 than in the near area. Because the imaging scale depends on the square of the tangent of slope angle .epsilon., the enlargement of the imaging scale is especially efficient when this slope angle .epsilon. is chosen to be larger than 45.degree., which is quite feasible even with wide-angle lenses having wide angle characteristics which need not be considered extreme. In commercial vehicles with a tall cab in which the front end of the vehicle is essentially defined by the vertical or approximately vertical plane of the windshield, the use of a camera lens with a total aperture angle of 90.degree. allows the camera to be mounted in the interior of the cab, and nevertheless to acquire the road over nearly its entire length between the vehicle and the horizon.
In addition, it is advantageous for the horizontal minimum distance from the camera lens (at which the latter casts the image of the acquired area on the photosensitive surface of the camera) to be approximately equal to the value h/tan.epsilon., where h is the vertical distance of the center of the lens from the road and .epsilon. is once again the slope angle that the optical axis of the lens forms with the road.